Lufthansa Chief Executive Carsten Spohr (C) and Germanwings Managing Director Thomas Winkelmann (2ndL) speak during a news conference near the memorial for the victims of the air disaster in the village of Le Vernet, near the crash site of the Germanwings

Lufthansa's boss declined to answer a barrage of questions on Wednesday on what the airline knew of the mental health problems of Andreas Lubitz, the pilot believed to have deliberately crashed a plane in the French Alps last week.

The German carrier said on Tuesday it had uncovered emails showing that Lubitz, who flew for Lufthansa's budget airline Germanwings, had told its flight school that he had suffered a period of severe depression.

The revelation added to questions about the adequacy of psychological testing for pilots, and about strict German doctor-patient confidentiality laws.

The chief executive, Carsten Spohr, declined to respond to intense questioning from journalists on Thursday after visiting a memorial site near the crash in France, walking away after giving a prepared statement.

Lubitz, who was allowed to restart training after passing all the medical and suitability checks again, had a note on his flight license indicating some sort of illness, a person familiar with the matter told Reuters.

But under German doctor-patient confidentiality laws, Lufthansa as an employer may not seek information about employees' medical conditions.

Lufthansa has said Lubitz was passed fit to fly by medical examiners at his last annual check.

Germany's Transport Ministry also said it would wait until investigations were complete before making any changes to air safety rules.

Video found

Spohr said in the prepared statement that it was not clear what drove Lubitz to steer the Airbus A320 into a remote mountainside, killing all 150 onboard.

"We are learning more every day about the causes of the accident," Spohr said. "It will take a long, long time for all of us to understand how this could happen."

The German newspaper Bild said a video showing the chaotic final seconds on board the Germanwings flight had been discovered near the site.

The crash has opened up Lufthansa to the prospect of legal claims from families of the victims.

Insurance sources have said that insurers will stand behind their liability policies for Lufthansa and Germanwings, despite Tuesday's revelation.

Germany's Allianz, which is coordinating the insurance response, said all claims arising from the crash would be handled fully, fairly and as quickly as possible.

The insurer's puts the total cost of the crash at $300 million, but has said the figure could rise or fall as new information becomes available.

"The reserve figure has no effect on how individual claims will be evaluated and assessed," an Allianz spokeswoman said.

Spohr said the airline would continue to provide assistance to the victims' families for as long as they need.